1. Field of the Invention
This invention relates to a ferroelectric liquid crystal display, and more particularly to a pressure sealing apparatus and method for a ferroelectric liquid crystal display that is adaptive for improving an alignment characteristic of a liquid crystal.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls light in accordance with a liquid crystal alignment state to thereby display a desired picture on the screen. A liquid crystal used for such an LCD is in a neutral phase between a liquid state and a solid state, which has both a fluidity and an elasticity. In a thermodynamic phase transition process of the liquid crystal, a liquid crystal having a smectic C phase is rotated along a smectic layer taking a layer structure having the same electrical and magnetic property. In other words, the smectic C phase liquid crystal is rotated along an outer line of a virtual cone.
Such a smectic C phase liquid crystal has a characteristic of making a spontaneous polarization regardless of an external electric field. This liquid crystal is usually referred to as ‘ferroelectric liquid crystal’ (FLC). The FLC has been actively studied in light of a fast response speed according to its spontaneous polarization characteristic and an ability to realize a wide viewing angle without a special electrode structure and a compensating film. The FLC includes a deformed helix FLC mode, a surface stabilized FLC mode, an anti-FLC mode, a V-type FLC mode and a half V-type FLC mode, etc. Hereinafter, the V-type FLC mode and the half V-type FLC mode in such modes will be described.
FIG. 1 shows an alignment state of a liquid crystal cell in the V-type FLC mode.
Referring to FIG. 1, the liquid crystal cell in the V-type FLC mode includes an upper substrate 1 on which a common electrode 3 and an alignment film 5 are disposed, a lower substrate 11 on which a TFT array 9 including a pixel electrode and an alignment film 7, and a liquid crystal 13 injected between the upper and lower substrates 1 and 11. The alignment films 5 and 7 formed on the upper substrate 1 and the lower substrate 11, respectively, are aligned in the horizontal direction by the rubbing. Subsequently, the liquid crystal 13 is injected between the upper substrate 1 and the lower substrate 11 after joining of the upper substrate 1 to the lower substrate 11. The injected liquid crystal 13 forms a smectic layer taking a layer structure and is arranged into a phase having a desired slope with respect to a plane perpendicular to the smectic layer. In other words, the liquid crystal 13 has a desired inclination angle with respect to a horizontally aligned direction of the alignment film and is aligned such that the adjacent smectic layers have opposite polarities with respect to each other.
A transmittance according to a voltage of the V-type FLC mode liquid crystal cell is indicated in FIG. 2. The liquid crystal 13 within the V-type FLC mode liquid crystal cell responds to positive and negative voltages applied thereto. Since the transmittance is suddenly changed according to an application of the positive and negative voltages, a transmittance curve according to a voltage has roughly a V-shape. In other words, a transmittance is increased, independently of the polarities, as a positive voltage increases.
FIG. 3 shows an alignment state of a liquid crystal cell in the half V-type FLC mode.
In FIG. 3, a liquid crystal 15 within the half V-type FLC mode liquid crystal cell injected between the upper substrate 1 and the lower substrate 11 forms a smectic layer taking a layer structure. The liquid crystal 15 is aligned at a desired inclination angle with respect to a horizontal aligned direction of the alignment films 5 and 7 such that the adjacent smectic layers have a different polarity unlike the liquid crystal 13 in the V-type FLC mode. Such a half V-type mode liquid crystal can be implemented by applying a positive or negative electric field in advance and, at the same time, lowering its temperature into a temperature having a smectic phase. The half V-type FLC mode liquid crystal 15 formed in this manner responds to only one of the applied positive and negative voltages. Thus, as seen from FIG. 4, a transmittance curve according to a voltage of a liquid crystal cell in the half V-type FLC mode has a ‘half V’ shape. A T-V characteristic in FIG. 4 represents when a negative voltage is used to make an initial uniform alignment. In this case, a transmittance appears to not increase upon application of a negative voltage, whereas it is increased as a positive voltage increases. Similarly, when a positive voltage is used to make an initial uniform alignment, a transmittance is increased as a negative voltage increases.
A thermodynamic phase transition process of the half V-type FLC mode liquid crystal 15 is as follows:                Isotropic→nematic (N*) phase→smectic C* (Sm C*) phase→crystal        
Such a phase transition process expresses a liquid crystal phase resulting from a gradual decrease in temperature as moving to the right. The liquid crystal 15 is aligned in parallel to a rubbing direction when its temperature is slowly lowered to reach a temperature having a nematic phase after the liquid crystal 15 was injected into the liquid crystal cell at a temperature having an isotropic phase. If an electric field is applied to the interior of the cell with a temperature being slowly lowered in this state, then the liquid crystal 15 is phase-changed into a smectic phase. A direction of a spontaneous polarization of the liquid crystal 15 generated at this time is arranged in such a manner to be consistent with that of an electric field formed at the interior of the cell. As a result, when the liquid crystal 15 within the liquid crystal cell is subjected to a parallel alignment treatment, it makes one of two possible molecule arrangements. The molecule arrangement in the spontaneous polarization direction is consistent with the direction of an electric field applied in the phase transition process, and thereby has a uniform alignment state.
This will be described in detail with reference to FIG. 5 and FIG. 6 below.
First, as seen from FIG. 5, if a negative electric field E(−) is applied upon alignment of the liquid crystal 15, then a spontaneous polarization direction of the liquid crystal 15 identical to the electric field direction is made to provide a uniform alignment. In such a liquid crystal cell, as shown in FIG. 6, a liquid crystal arrangement is changed upon application of a positive electric field E(+) while it is not changed upon application of a negative electric field E(−). In order to utilize a response characteristic to an electric field of the liquid crystal 15, polarizers perpendicular to each other are arranged at the upper and lower portions of the liquid crystal cell. At this time, a transmission axis of one polarizer is arranged to be consistent with an initial liquid crystal alignment direction. In the liquid crystal cell taking the above-mentioned arrangement, a transmission curve according to a voltage application has a ‘half V’ shape as shown in FIG. 4. With respect to a negative electric field E(−), a liquid crystal arrangement is not changed to shut off light. Otherwise, with respect to a positive electric field E(+), a liquid crystal arrangement is changed to transmit light. In this case, as a positive electric field E(+) increases, a transmittance increases.
As described above, the half V-type FLC mode liquid crystal utilizes both a temperature and an electric field so as to obtain a uniform alignment.
A method of fabricating LCD's including the ferroelectric LCD is classified into substrate cleaning, substrate patterning, alignment film formation, substrate joining/liquid crystal injection, packaging and test processes.
In the substrate cleaning process, a cleaner removes an alien substance on the substrates before and after patterning of the upper and lower substrates.
The substrate patterning process is divided into a step of patterning the upper substrate and a step of patterning the lower substrate. The upper substrate is provided with color filters, a common electrode and black matrices, etc. The lower substrate is provided with signal wires such as data lines and gate lines, etc. A thin film transistor (TFT) is arranged at each intersection between the data lines and the gate lines. A pixel electrode is formed at each pixel area between the data lines and the gate lines.
In the alignment film formation process, an alignment film is coated on each of the upper substrate and the lower substrate and then rubbed.
In the substrate joining/liquid crystal injection process, the upper substrate is joined to the lower substrate after a sealant is coated onto the lower substrate. Subsequently, a liquid crystal is injected between the upper and lower substrates through a liquid crystal injection hole provided at one side of the sealant. Thereafter, the liquid crystal injection hole is sealed.
A cell gap of the liquid crystal display panel may become non-uniform due to the liquid crystal injected between the upper and lower substrates. A non-uniformity of the cell gap increases as a viscosity of the liquid crystal increases, as a liquid crystal injection time increases, or as a gap of the liquid crystal display panel decreases. In order to prevent such a non-uniformity of the cell gap that may occur in the substrate joining/liquid crystal injection process, a pressure-sealing process of pressurizing the liquid crystal display panel injected with the liquid crystal at an entirely uniform pressure and then sealing the liquid crystal injection hole using an apparatus as shown in FIG. 7 is employed.
FIG. 7 shows a conventional pressure sealing apparatus.
Referring to FIG. 7, the conventional pressure sealing apparatus includes an upper pressurizing plate 101 for pressurizing the liquid crystal display panel downwardly, and a lower plate 102 for supporting the pressurized liquid crystal display panel from the lower portion thereof.
The liquid crystal display panel includes an upper substrate 103 on which a common electrode 111 and an upper alignment film 104 are disposed; a lower substrate 107 on which a pixel electrode 110 and a lower alignment film 106 are disposed; and a liquid crystal 105 injected between the upper substrate 103 and the lower substrate 107. Further, sealants 108 and 109 and spacers (not shown) for defining a gap so that the liquid crystal 105 can be injected are provided between the lower substrate 107 and the upper substrate 103 of the liquid crystal display panel.
After the liquid crystal injection, the liquid crystal display panel is put on the lower plate 102 of the pressure sealing apparatus. The liquid crystal display panel on the upper plate 102 is pressurized at a constant pressure by the upper pressurizing plate 101. Then, a uniform pressure is applied to the entire liquid crystal display panel by a pressure from the upper pressurizing plate 101, thereby allowing a cell gap of the liquid crystal display panel to be uniform. When the cell gap of the liquid crystal display panel becomes entirely uniform, the liquid crystal injection hole is sealed with the sealant 108, which is a photo hardening resin.
However, the conventional pressure sealing process is suitable for a nematic phase liquid crystal which has a low viscosity and remains in a liquid state, but has a problem in that it is difficult to be applied to a ferroelectric liquid crystal having a smectic phase that keeps a high viscosity gel state at a normal temperature. If a pressure is applied to a high viscosity liquid crystal, then an interface of the alignment film aligned at a certain inclination angle by the rubbing process is destroyed or deformed, thereby causing a deformation in an alignment property of the liquid crystal.
Meanwhile, the ferroelectric liquid crystal has a problem of thermal contraction according a thermal expansion coefficient when the liquid crystal is phase-changed from a nematic phase into a smectic phase. Accordingly, in the case of pressurizing a ferroelectric liquid crystal, it is necessary to pressurize the ferroelectric liquid crystal while slowly lowering a temperature of the liquid crystal from a temperature at which the liquid crystal has the largest bulk.
Furthermore, a half V-type FLC mode liquid crystal is subjected to a temperature treatment and an application of an electric field so as to obtain a uniform alignment. The half V-type FLC mode liquid crystal has a problem in that, since it becomes more difficult to uniformly apply a large-area liquid crystal layer to the electric field and the temperature treatment because the LCD has a larger area, it is difficult to pressure seal a ferroelectric liquid crystal layer having a uniform alignment characteristic at a large area.